Most common locks are pin-tumbler cylinder locks or wafer-tumbler cylinder locks. Each of these locks contains a cylinder or plug which rotates within a housing or shell. In the pin-tumbler locks, pin holes containing top and bottom pin tumblers extend transversely through both the cylinder and the housing, and may be crossing the shear line, i.e. the boundary between the cylinder and the housing. The pin tumblers slide up and down within the pin holes defining a locked position and an unlocked position. When a pin tumbler crosses the shear line, the pin tumbler interferes with the rotation of the cylinder and the cylinder remains locked. When the correct key is inserted in the lock, the bitting on the key contact the pin tumblers and force them to slide within their pin holes such that no pin crosses the shear line. The cylinder can then rotate within the housing when a torque is applied by the key to unlock the lock.
Wafer-tumbler locks have wafer-shaped tumblers which slide up and down within channels that extend from the cylinder to the housing. The wafer tumblers are spring loaded so that they can extend out of the cylinder and into a locking slot within the housing, thereby preventing rotation of the cylinder relative to the housing in a locked position. The center of each of the wafer tumblers has an opening for receiving a key. The correct key moves the wafer tumblers out of the locking slot, such that torque applied to the cylinder rotates the cylinder within the housing and unlocks of the lock.
To avoid or reduce the costs of changing or re-keying locks in office and apartment buildings, for example, several types of re-keyable locks that do not require disassembly have been developed for pin-tumbler locks, see, for example, U.S. Pat. Nos. 4,412,850 and 5,233,850. Simple and cost-effective re-keyable locks for wafer tumbler systems are still needed.